The present invention relates to mass transfer between fluids and, in particular, to mass transfer operations performed on blood. Such mass transfer operations may, for example, include plasmapheresis, hemodialysis, the displacement of water with cryoprotective agents, such as glycerine or ethylene glycol to provide blood for freezing, and the displacement of plasma from whole blood or of cryoprotective agents from thawed blood with saline solution to provide "washed" blood cells.
In the aforementioned copending applications Ser. No. 523,007 and Ser. No. 809,923, applicant has disclosed continuous systems for plasmapheresis, utilizing unique fractionation devices which are highly efficient and compact. Nevertheless, it is always an object to minimize the size of fractionation devices, for reasons of economy of manufacture, to facilitate portability in use and to increase the filtration rate achievable with the device, which is proportional to the length of the flow path through the device. But, in general, further reduction of the size of the device decreases flow rate per unit area. Thus, it is necessary to increase the effective flow rate in order to realize the benefits of increased filtration rate. But the flow rate achievable in continuous loop systems connected to a human donor are limited, since peripheral vein flow in humans is limited to a maximum of 80 to 100 ml/min. Thus, for the particular design of device disclosed in those copending applications, significant size reduction beyond that disclosed was not feasible.
Certain operations, such as plasmapheresis and hemodialysis, are performed on blood as it is received from a human source. In these operations a substance is removed from the blood (plasma in the case of plasmapheresis and blood impurities in the case of hemodialysis), and the remaining constituents are returned to the human source. This typically requires the connection of the apparatus to the human in a closed loop. Originally this was done by the use of two needles inserted into the human, one for withdrawing blood from the human and the other for returning blood to the human. But this is an undesirable arrangement, since most persons dislike having to receive two needles.
It is known to use a double-lumen needle so that only one needle need be used. But this type of needle has given rise to additional problems. In some cases there is insufficient blood flow because the inlet port of the needle becomes occluded by the vein wall. Also, the use of this technique may result in excessive recirculation of blood from the return lumen back into the withdrawal lumen. In the case of plasmapheresis, this may result in undue dilution of the blood by anticoagulant added in the plasmapheresis process.
In the case of hemodialysis, it is also known to utilize a single needle with a single lumen by utilizing valves in the two legs of the tubing loop which lead to and from the needle, and alternately opening and closing these valves, so that blood alternately flows in opposite directions through the needle. But such prior systems have had a number of disadvantages. The system pump runs all the time and, since the blood is flowing from the human source only half the time, the pump must run at twice the normal rate in order to achieve the same flow rates that would be obtained with the two-needle or double-lumen needle approaches. Furthermore, during those times when the valves are configured so that the blood is flowing back to the human, there is no flow from the human, so that, essentially, the pump is operating with an open input. This can result in collapsed input tubing and the creation of air bubbles in the lines which, in turn, necessitates the use of air traps on both sides of the dialyzer. These air traps store a considerable volume of blood and significantly increase the extracorporeal blood volume of the system.
In the case of operations such as blood freezing and washing, which may not require connection to a human donor, the process of displacement of water or cryoprotective agents from the blood is quite time consuming. The rate of addition of saline or cryoprotective agent to the blood cells must be carefully balanced with the rate of removal of water or cryoprotective agent, since otherwise the blood cells are damaged by collapse or expansion. In existing systems, the process of monitoring and regulating the flows are all manually controlled, requiring constant attendance by trained personnel.